1. Field of the Invention
The present invention relates to a ball for use in a constant-velocity joint such as a constant-velocity joint for use in automobiles, industrial machines, or the like, especially a constant-velocity joint for use in front and rear axles of automobiles, the ball being capable of increasing the service life of the constant-velocity joint, and a method of manufacturing such a ball.
2. Description of the Related Art
As shown in FIGS. 1 and 2 of the accompanying drawings, a constant-velocity joint 1 comprises an inner race 5, a cage (retainer) 7, an outer race 3, and a plurality of balls 6, and transfers a torque between the balls 6 and outer race grooves 3b and inner race grooves 5b. Recently, constant-velocity joints are required to be more compact because of the demand for high torque or weight reduction. However, small-size constant-velocity joints tend to have poor durability. Japanese laid-open patent publication No. 2000-145804 discloses ball rolling grooves in outer and inner races which are processed into a surface roughness ranging from 10 to 20 xcexcm by shot blasting, then chemically treated to provide a surface layer of manganese phosphate or zinc phosphate, and coated with a solid lubricant film of molybdenum disulfide and graphite which are bonded by a resin. Japanese laid-open patent publication No. 6-173967 discloses ball rolling grooves having a hardened layer (nitrided layer) of quenched martensite structure with a solid solution of nitrogen.
Balls used in general constant-velocity joints are substantially identical to balls used in standard rolling bearings except that the former balls are slightly less accurate than the latter balls. The balls used in general constant-velocity joints are made of bearing steel (SUJ2: JIS G 4805) or an equivalent material. They are produced by hardening the material at a temperature ranging from 820 to 850xc2x0 C. and then tempering it at a lower temperature ranging from 150 to 180xc2x0 C. to provide a surface hardness of at least HRC (Rockwell hardness C scale) 62. According to Japanese laid-open patent publication No. 2000-145805, the diameter of balls is decreased to use an increased number of balls. However, since the number of balls used is increased, it is necessary to make efforts to assemble the balls neatly and design a retainer for holding those many balls. Japanese laid-open patent publication No. 2000-74082 discloses the use of steel balls that are heat-treated to stabilize heat-resistant dimensions in order to keep a remaining amount of austenite at 5% or less and make the surface hardness in the range from HRC 53 to 61. The disclosed steel balls are resistant to heat at high rotational speeds and at high angles, and are prevented from being deformed due to aging. Therefore, since the surface hardness is in the range from HRC 53 to 61 which is lower than general surface hardness ranges, the balls are low in durability. Furthermore, because the remaining amount of austenite is low, the surface of the balls is weak, and hence the balls tend to have a short service life. In addition, the balls used in constant-velocity joints are problematic in that since the balls have a cross hatch angle and make a composite motion including slipping and rolling motions, the surface of the balls tend to be damaged due to oil film interruptions thereon and peeled off.
According to Japanese patent publication No. 3047088, in order to increase the wear resistance and fatigue strength of balls for constant-velocity joints, the surface of the balls is carbonitrided and then tempered at 170xc2x0 C. for 2 hours to achieve a hardness ranging from about HV 800 to 860 (from about HRC 64 to 66). However, since the carbonitrided surface has a high carbon concentration and is liable to be crushed under a reduced load, the balls tend to be damaged under shocks applied when the automobile with such constant-velocity joints runs on a gravel road or hits a curb.
The inventors of the present invention have attempted to apply the idea of the nitriding of ball rolling grooves as disclosed in Japanese laid-open patent publication No. 6-173967 to balls as taught by Japanese patent publication No. 3047088. However, when nitrided, balls become more liable to be crushed under a reduced load, and hence tend to crack and be damaged easily.
It is therefore a major object of the present invention to provide a ball for use in a constant-velocity joint, which is less subject to surface damage and has increased durability and hence a long service life, and a method of manufacturing such a ball.
According to the present invention, a ball for use in a constant-velocity joint, which is adapted to be interposed between an inner race and an outer race for transmitting rotational power therebetween, is made of bearing steel or a material equivalent thereto, and has a surface nitrided and processed for increased resistance to a crushing load.
The strength of the surface of the ball is increased by nitriding the surface of the ball to increase the remaining amount of austenite on the surface of the ball. When the surface of the ball is processed for increased resistance to a crushing load, it is prevented from suffering a reduction in the crushing load due to the nitriding process. The ball thus produced is resistant to surface damage and a high surface pressure, and has a high strength. The crushing load refers to a vertical load under which either one of two balls of the same nominal diameter superposed on a conical seat having an angle of 120xc2x0 is broken, as defined in JIS B 1501, revised in 1983, Reference 3.
Since the resistance to the crushing load appears on the surface of the ball, the surface hardness of the ball may be adjusted to be in the range from HRC 60 to 64. If the surface hardness of the ball were smaller than HRC 60, then the surface hardness of the ball would be too small, and if the surface hardness of the ball were greater than HRC 64, then the crushing load would be too low.
The ball may be processed for increased resistance to a crushing load by lowering the hardening temperature or lowering the tempering temperature. According to the present invention, it is preferable to nitride the ball in an atmospheric temperature of 840xc2x0 C., quench the ball in oil (after being nitrided or hardened by nitridation), and temper the ball in a temperature range from 180 to 230xc2x0 C. Since the surface hardness of the ball which has excessively been increased by the nitridation is lowered by tempering the ball at a temperature higher than a general tempering temperature, the strength of the ball against the crushing load can be increased. If the tempering temperature were lower than 180xc2x0 C., then the surface hardness of the ball would remain so high that the ball would be too weak under a crushing load. If the tempering temperature were higher than 230xc2x0 C., then the surface hardness of the ball would become too low to make the ball resistant to surface damage.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.